Risers are normally used to link hydrocarbon wells on the seabed to offshore floating structures. A riser is normally made up of lengths of tubing of steel having significant diameter, making them heavy. The floating structure therefore needs to apply tension to the riser to prevent it from buckling and possibly to collapse under its own weight, and prevent the weight from acting on the wellhead. This tension system is also compensated for movements of the platform relative the seabed, e.g. to keep a relative steady tension in the riser. Problems may occur when the platform experiences conditions out of normal operation range such as drive-off and drift-off, or if the heave compensation system is not working properly. All these conditions may result in excessive tension in the riser, and at some point the riser will break. To address this problem, risers may be provided with a weak link which has a lower tensile rating than the other components of the riser such that one gets a breakage at a given/predetermined point in the riser when there is a given tension in the riser, known prior to the incident.
A weak link shall comply with the following requirements:                Protect barriers, both primary and secondary        Protect personnel        Protect environment        
A conventional weak link comprises two parts which are releasable attached to one another by, for example, studs, which fracture at a predetermined tensile force. Such conventional weak link systems shall be able to withstand tensile forces applied to the weak link not only by the offshore structure, but also by well pressure. The studs therefore have to be rated to separate at a tension which is a combination of the separation force supplied by the well pressure and the tension applied from surface. The well pressure fluctuates. At high well pressures a conventional weak link can provide a very limited operational utilization as it will require a very limited external tension before it breaks, and at low pressures a conventional weak link can fail to protect the system as it will require a relatively higher external tension before it breaks. This might be a problem, both with regards to the operational window, but also in relation to safe protection of existing equipment at the wellhead, such as the barrier within the well.
Another element with standard weak links is that breaking a weak link in a riser due to excessive tension, e.g. as a result of drive-off, drift-off or sudden raise in the fluid pressure within the riser, will release massive forces which will act on the riser giving the riser an undesired behaviour. If the riser breaks, due to excessive tension, the riser will act like a pulled-out spring and may, in a worst case scenario, shoot out of the water like a projectile towards the offshore structure and cause severe damage to personnel and/or the structure/platform. Another problem may be that if the weak link and/or riser connection break, entrapped gas or hydrocarbons may be released to the sea or surface. In such situations it is desirable to be able to control the behaviour of the riser and the riser content, and, possibly perform a controlled disconnect. Different solutions have been used in the technical fields of weak links and pressure compensated riser connections, including EP 2310613, U.S. Pat. No. 8,181,704, U.S. Pat. No. 5,382,052, U.S. Pat. No. 4,361,165 and U.S. Pat. No. 4,059,288.
An objective of the present invention is therefore to provide a safety joint that limits the problems related to prior weak links and allow for a larger operational envelope compared to traditional weak links.